Apparatus and method for liquid level measurement in electrolytic cells

ABSTRACT

An apparatus and method for detecting a liquid level in an electrolytic cell are disclosed herein, the apparatus comprising a level tube in fluid contact with the electrolytic cell; a proximity sensor positioned to detect the presence or absence of liquid at a predetermined level in the level tube; and a control system responsive to the proximity sensor, wherein the control system is in communication with the liquid level sensor via a communication system. The proximity sensor detects the presence or absence of fluid in the level tube and sends a signal to the control system via the communication system; and the control system provides an indication of liquid level.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/600,146 filed on Feb. 17, 20012, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to an apparatus and methodfor detecting a liquid level in an electrolytic cell.

Low or high liquid levels in electrolytic cells may result fromnon-equilibrium conditions present during shut-down and start-up, aswell as from problematic cells having “fast running” diaphragms. A lowliquid level in an electrolytic cell, coupled with a failure to detectthe low liquid level in a timely manner, can result in a potentiallyhazardous situation. For example, when the electrolytic process is thechlor-alkali process, and the electrolytic cell is a diaphragm cell, adecrease in the liquid level below the low-low liquid level can resultin exposure of the tops of the cathodes to the head space gases of theelectrolytic cell, which comprise chlorine gas. When a portion of thesurface of the cathode is exposed to the head space gases, thepermeability of the diaphragm to hydrogen gas increases, and thepotential exists for leakage of hydrogen gas into the head spacecomprising chlorine gas. When the amount of hydrogen gas in the headspace is about 3 weight percent or more, there is a risk of anexothermic reaction of hydrogen with chlorine to form hydrogen chloride,i.e., a hydrogen fire caused by oxidation of hydrogen by chlorine. Ifthe hydrogen concentration is high enough, detonation can also occur.Thus it is desirable to have an apparatus and method for detectingliquid level in an electrolytic cell wherein the liquid level can becontinuously monitored from a remote location.

Liquid level can be measured in opaque electrolytic cells using a levelgauge. The simplest and most reliable level gauge is a liquid sightmonitor, also known as a direct reading level gauge, or sight glass.Under normal conditions, the electrolytic cells can be inspected by anoperator at approximately 30-minute intervals. However, this inspectioninterval may be insufficient to catch “fast running” cells, in which theliquid level can decrease to a dangerously low level in less than 30minutes.

Many methods for liquid level measurement other than direct visualobservation are known. These include the use of float gauges, in which afloat rests on the surface of the liquid, and is magnetically oroptically coupled to a sensor; and the use of sensors that operate bymeasuring electrical resistance in the liquid. However, these methodsare invasive, i.e., they require a float or a sensor to be in directcontact with the liquid. This is undesirable where the liquid in anelectrolytic cell is corrosive. For example, the liquid in the anodehalf-cell in the chlor-alkali process contains chloride and dissolvedchlorine gas, and is therefore highly corrosive. Moreover, the intensemagnetic fields, arising from the application of high voltage, that arepresent during operation of electrolytic cells can interfere with theoperation of these liquid level sensors. These harsh conditions can leadto malfunction of the sensors, and even render the sensors inoperable,which makes it difficult to know if the signals generated by the sensorsare representative of the actual liquid level in an electrolytic cell.Moreover, the installation of these sensors requires modification of theelectrolytic cell, as well as the down-time and capital investmentnecessary to make the modifications.

For at least these reasons, liquid level sensors in current use can beunreliable or lack cost-effectiveness under the operating conditions ofelectrolytic cells. It is therefore desirable to have an apparatus andmethod for detecting a liquid level in an electrolytic cell withimproved reliability, particularly under severe operating conditionssuch as high magnetic fields and corrosive atmospheres. It would be afurther advantage if installation does not require modification ofexisting electrolytic cells. It would be a still further advantage ifthe apparatus and methods are cost-effective.

BRIEF DESCRIPTION OF THE INVENTION

An embodiment is an apparatus for detecting a liquid level in anelectrolytic cell, the apparatus comprising a level tube in fluidcontact with the electrolytic cell; a proximity sensor positioned todetect the presence or absence of liquid at a predetermined level in thelevel tube; and a control system responsive to the proximity sensor,wherein the control system is in communication with the liquid levelsensor via a communication system.

Another embodiment is an apparatus for detecting a brine level in achlor-alkali electrolytic cell, the apparatus comprising a level tube influid contact with the chlor-alkali electrolytic cell; a capacitiveproximity sensor adjustably mounted on the level tube to detect thepresence or absence of brine at a predetermined position on the leveltube; a support for positioning and mounting the capacitive proximitysensor on the level tube; and a digital control system responsive to thecapacitive proximity sensor, wherein the digital control system is incommunication with the capacitive proximity sensor.

Another embodiment is a method for maintaining a target liquid level inan electrolytic cell, the method comprising providing an apparatuscomprising a level tube in fluid contact with an electrolytic cell, aproximity sensor positioned on the level tube at a target liquid levelto detect the presence or absence of a liquid, and a control systemresponsive to the proximity sensor, wherein the control system is incommunication with the proximity sensor via a communication system, andindicates the presence or absence of the liquid at the target liquidlevel on the level tube in response to a signal from the proximitysensor; and adjusting the liquid level in the electrolytic cell to thetarget liquid level in response to the signal.

Another embodiment is a method for maintaining a target brine level in achlor-alkali electrolytic cell, the method comprising providing anapparatus comprising a level tube in fluid contact with the chlor-alkalielectrolytic cell, a capacitive proximity sensor adjustably mounted onthe level tube to detect the presence or absence of brine at the targetbrine level on the level tube, a support for positioning and mountingthe capacitive proximity sensor on the level tube, and a digital controlsystem responsive to the capacitive proximity sensor, wherein thedigital control system is in communication with the capacitive proximitysensor and indicates the presence or absence of the brine at the targetbrine level on the level tube in response to a signal from thecapacitive proximity sensor; and adjusting the brine level in thechlor-alkali electrolytic cell to the target brine level in response tothe signal.

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other features and advantages are apparent from thefollowing detailed description, claims, and examples, taken inconjunction with the accompanying Figure, which is not limiting, and inwhich:

The Figure is a schematic diagram of an apparatus for detecting liquidlevel in an electrolytic cell comprising a level tube in fluid contactwith the electrolytic cell and a proximity sensor.

DETAILED DESCRIPTION OF THE INVENTION

The inventors hereof have developed an apparatus and method fordetecting a liquid level in an electrolytic cell. Accordingly, a leveltube is placed in fluid contact with the electrolytic cell to detect thelevel of the liquid in the cell. The level tube is fitted with aproximity sensor positioned to detect the presence or absence of liquidat a predetermined level in the level tube. A communication system linksthe proximity sensor to a control system responsive to the proximitysensor. The control system can be used to alert the operator when thefluid level of the electrochemical cell is not within the desired range,or to perform other actions.

Thus, a method for detecting a high or low liquid level in anelectrolytic cell comprises use of an apparatus comprising a level tubein fluid contact with an electrolytic cell, a proximity sensorpositioned on the level tube to detect the presence or absence ofliquid, and a control system responsive to the proximity sensor, whereinthe control system is in communication with the proximity sensor via acommunication system; and wherein the proximity sensor detects thepresence or absence of fluid in the level tube and sends a signal to thecontrol system via the communication system. The control system can thenprovide an indication of liquid level in response to the signal from theproximity sensor. In a method for maintaining a target liquid level inan electrolytic cell, the liquid level in the electrolytic cell isadjusted to the target level in response to the signal from theproximity sensor to the control system.

Advantageously, the apparatus and methods are reliable under severeoperating conditions such as high magnetic fields and corrosiveenvironments. Moreover the apparatus and methods do not requiremodification of existing electrolytic cells, are cost-effective , and donot require process down-time or large capital investment to implement.

The terms “a” and “an” do not denote a limitation of quantity, butrather the presence of at least one of the referenced item. The term“or” means “and/or.” The open-ended transitional phrase “comprising”encompasses the intermediate transitional phrase “consisting essentiallyof” and the close-ended phrase “consisting of” Claims reciting one ofthese three transitional phrases, or with an alternate transitionalphrase such as “containing” or “including” can be written with any othertransitional phrase unless clearly precluded by the context or art.Recitation of ranges of values are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. The endpoints of all ranges are includedwithin the range and independently combinable. Reference throughout thespecification to “another embodiment,” “an embodiment,” “someembodiments,” and so forth, means that a particular element (e.g.,feature, structure, property, and/or characteristic) described inconnection with the embodiment is included in at least one embodimentdescribed herein, and can or cannot be present in other embodiments. Inaddition, it is to be understood that the described element(s) can becombined in any suitable manner in the various embodiments.

The Figure is a schematic diagram of an apparatus 1 for detecting liquidlevel (the position of the liquid-gas interface) in an electrolytic cell11 comprising a level tube in fluid contact with a liquid contained inthe electrolytic cell 11 and a proximity sensor 3. In particular, in theFigure, electrolytic cell 11 contains liquid 12 exhibiting a liquidlevel 8 in electrolytic cell 11. Liquid 12 can be any liquid in thecell. In an embodiment, it is an electrolyte, for example a solution ofwater in which ions are dissolved. In an embodiment, the liquid is asolution of sodium chloride in water (brine). The liquid level 8 in thecell can vary depending upon process conditions. The liquid level can beadjusted, for example by increasing or decreasing the flow of liquidinto or out of the electrolytic cell. Level tube 2 (also known as adirect reading level gauge, or sight glass) is in fluid contact with theliquid 12 in the electrolytic cell so that the liquid level 8 in thelevel tube 2 is representative of the liquid level 8 in the electrolyticcell. Proximity sensor 3 is positioned adjacent the level tube 2 so asto detect liquid level 8 in the level tube. Measurement zone 5represents the area in which the proximity sensor detects the presenceor absence of liquid. A variety of means can be used to position theproximity sensor 3. For example, the proximity sensor can be imbedded inor attached to a support 7 as shown in the Figure, which is held inplace by a clamp, or an elastomeric O-ring 6 as shown in the Figure. Inan embodiment, the position of the proximity sensor is fixed, forexample by an adhesive. In other embodiments the position of theproximity sensor is adjustable along the vertical axis of the leveltube. Liquid level 9 represents a low liquid level, and liquid level 10represents for example a critical liquid level, at which there is anincrease risk of a hazardous condition. The proximity sensor cancomprise at least one display light, which is indicative of the presenceor absence of liquid at the position of the proximity sensor. Theproximity sensor 3 is in communication with a communication bus, forexample via electrical connection 4.

The electrolytic cell can be any chemical reactor that utilizes directcurrent electricity to induce the simultaneous oxidation and reductionof charged chemical components of an electrolyte (ions or a solutioncontaining ions) to produce oxidized and reduced products in a processcalled electrolysis. The electrolyte can be a solution of an inorganicsalt in a solvent, for example water. The electrolyte can also be amolten inorganic salt or oxide. Examples of electrolytes are aluminum,lithium, sodium, potassium, magnesium, calcium, copper, zinc, and leadsalts and/or oxides, which are used in the production of thecorresponding high purity metals. Specific examples of an electrolyteare bauxite, which is used in the production of aluminum, and sodiumchloride, which is used in the simultaneous production of chlorine gasand sodium hydroxide. In electrolysis, an electric voltage is applied tothe electrolyte via an anode and a cathode. The oxidation takes place ina half-cell comprising the anode, and the reduction takes place in ahalf-cell comprising the cathode. For example, in the chlor-alkaliprocess, the electrolyte is a solution of sodium chloride in water(brine). The chloride ions are oxidized to chlorine gas in the half-cellcomprising the anode, and water is reduced to hydrogen gas and hydroxideion in the half-cell comprising the cathode. The brine that bathes theanode, and in which oxidation takes place, is called the anolyte, andthe caustic solution which bathes the cathode, and in which reductiontakes place, is called the catholyte. In the diaphragm cell process, thetwo half-cells are separated by a permeable diaphragm, which can be madeof asbestos or a ceramic material, which allows migration of sodium ionsfrom the anode half-cell to the cathode half-cells, but not theback-migration of hydroxide ions. Thus in some embodiments, theelectrolytic cell is a chlor-alkali electrolytic diaphragm cell, and theliquid is brine. A schematic diagram of an example of a chlor-alkalielectrolytic diaphragm cell is provided in the Figure.

The level tube comprises a vertical tube that can be opaque,translucent, or transparent, generally constructed of glass or plastic,positioned outside of the electrolytic cell. The level tube is in fluidcommunication with the electrolytic cell so that the liquid level in thelevel tube is the same as the liquid level in the electrolytic cell.With reference to the Figure, the level tube 2 is in fluid contact withthe liquid 12 at the side of the electrolytic cell. The level tubeconnects to the electrolytic cell at a point near the tops of thecathodes and anodes. The dimensions of the level tube can vary widelydepending on the requirements of the application, manufacturingconsiderations, desired visibility of the liquid in the tube, and likeconsiderations. For example, the level tube can have an inside diameterof about 0.2 to about 2 inches, specifically about 0.25 to about 1 inch.The level tube can extend in a vertical direction about 6 to about 30inches, specifically about 10 to about 20 inches above the tops of thecathodes and anodes.

Although an operator can determine the liquid level in the tank simplyby viewing the liquid level in the level tube at various time intervalswhen the level tube is transparent, as described above, such monitoringis prone to error, and may still not provide adequate warning of a low-or high-level condition if the condition occurs between views.Accordingly, continuous automated monitoring of liquid level, optionallyat a remote location, is provided. Continuous automated monitoring ofliquid level also allows automated process control for shutting down theelectrolytic cell in an emergency situation or for adjusting the liquidlevel in electrolytic cells.

The inventors hereof have found that continuous monitoring can beprovided where the liquid level is detected with a proximity sensorpositioned adjacent to the level tube. In an embodiment, the proximitysensor is located on, and in contact with, an outside surface of thelevel tube. Proximity sensors detect the presence of nearby objectswithout any physical contact with the object. In the apparatus andmethod for detecting a liquid level in electrolytic cells, the proximitysensor detects the presence or absence of liquid at its position on thelevel tube, without coming into physical contact with the liquid. Theproximity sensor can send a signal indicating either the presence orabsence of liquid at its position on the level tube to the controlsystem via a communication system.

There are many types of proximity sensors, which operate using varioustypes of electromagnetic radiation or acoustic waves to detect targetobjects. Types of proximity sensors include, but are not limited to,capacitive sensors, eddy-current sensors, inductive sensors, magneticsensors, optical sensors, infrared sensors, and ultrasound sensors. Anyof the foregoing can be used. The proximity sensor can be in a “barrel”(cylindrical) shape or “flat” (planar) shape. Flat proximity sensors canhave the advantage of a lower weight than barrel proximity sensors,thereby imposing less stress on the level tube on which it is mounted.The proximity sensor can have a local display. The local display can bea LED or a LCD. A local display advantageously allows an operator toreadily determine the operational status of the sensor while visuallyconfirming the liquid level.

In some embodiments, the proximity sensor is a capacitive sensor. Thecapacitive sensor can have fixed or adjustable capacitance. In someembodiments, the capacitive sensor has adjustable capacitance. When thecapacitance is adjustable, the capacitance can be optimized for rapiddetection of the presence or absence of liquid at the position of thecapacitive sensor on the level tube. When the capacitance is fixed, thecapacitance should be such that it is suitable for detection of thepresence of absence of liquid. The capacitive sensor should beunaffected by any build-up of solid residue on the inside of the leveltube at the measurement position. Exemplary capacitive sensors that canbe used are available from Lion Precision of St. Paul, Minn., DwyerInstruments, Inc. of Michigan City, Ind. and others.

As stated above, the proximity sensor can be associated with the leveltube by a variety of means that provide the desired degree ofadjustability or fixedness. For example, as shown in the Figure, theproximity sensor rests on an elastomeric O-ring (6 in the Figure) aroundthe level tube and is reversibly affixed to the level tube by way of asupport (7 in the Figure). The proximity sensor is readily detached fromthe support and removed from the level tube. The position of the O-ringis readily adjustable by sliding along the vertical axis of the leveltube. Thus, the position of the proximity sensor along the vertical axisof the level tube can be easily adjusted. The electrical connection tothe proximity sensor can be fixed or adjustable and/or detachable. In anembodiment, the electrical connection is flexible so that the proximitysensor can be removed from the level tube for inspection and repairwithout disconnecting the electrical connection. Thus adjusting theposition of the proximity sensor on the level tube and maintenance ofthe proximity sensor can be done without interrupting the normaloperation of the electrolytic cell.

In a method of detecting the liquid level of an electrolytic cell, atarget liquid level is determined, for example, the liquid level thatprovides a good balance of productivity, efficiency, and safety. Thetarget liquid level depends on the type of electrolytic process and thetype of electrolytic cell being used. The target liquid level can beidentified with reference to any point within the electrolytic cell, forexample the top of the cell, or the vertical distance measured from thetop of the cathodes and anodes. When the electrolytic process is thechlor-alkali process, and a diaphragm cell such as the one schematicallydepicted in the Figure is used, the liquid level detected is that of theanolyte, i.e. the brine bathing the anode. The target liquid level canbe about 4 to about 12 inches, specifically about 5 to about 10 inches,and more specifically about 6 to about 8 inches, above the top of thecathode.

In some embodiments, a single proximity sensor is used to provide anindication of a low liquid level relative to the target liquid level.When the liquid level represents a low liquid level, the liquid levelcan be 0 to about 70%, specifically 0 to about 60%, more specifically 0to about 50% of the target liquid level. The liquid level can be 0 toabout 6 inches, specifically 0 to about 5 inches, and more specifically0 to about 4 inches, above the top of the cathode in the electrolyticcell. In some embodiments, a single proximity sensor is used to providean indication of a high liquid level relative to the target liquidlevel, for example the top of the cell. When the liquid level representsa high liquid level, the liquid level can be about 70 to about 95%,specifically about 70 to about 90%, more specifically about 80 to about90% of the target liquid level. The high liquid level can be about 6 toabout 3 inches, specifically 6 to about 4 inches, and more specifically6 to about 5 inches below the top of the electrolytic cell.

A plurality of proximity sensors can be positioned at various locationson the level tube. In some embodiments, two proximity sensors arepositioned on the level tube. In this configuration, the first proximitysensor can detect a first liquid level, and the second proximity sensorcan detect a second liquid level below the first liquid level. The twoproximity sensors can be used to provide indications of two liquidlevels, for example the first proximity sensor can be used to provide alow liquid level indication, and the second proximity sensor can be usedto provide a low-low liquid level indication. Indications of low liquidlevel and low-low liquid level are indications that action should betaken by an operator to increase the liquid level to the target liquidlevel, for example, by increasing the flow rate of the liquid into theelectrolytic cell. The second proximity sensor, which provides a low-lowliquid level indication, can serve as a back-up to the first proximitysensor, which provides a low liquid level indication.

When the first liquid level represents a low liquid level, the firstliquid level can be about 40 to about 80%, specifically about 50 toabout 70%, of the target liquid level. When the second liquid levelrepresents a low-low liquid level, the second liquid level can be 0 toabout 40%, specifically about 20 to about 40%, and more specificallyabout 30 to about 40%, of the target liquid level. The first liquidlevel can be about 2 to about 6 inches, specifically about 3 to about 5inches, and more specifically about 3.5 to about 4.5 inches above thetop of the cathode in the electrolytic cell. The second liquid level canbe 0 to about 4 inches, specifically about 1 to about 3 inches, and morespecifically about 2 to about 3 inches, above the top of the cathode inthe electrolytic cell.

A reduction in liquid level in the electrolytic cell below the low-lowlevel described above is a potentially hazardous condition. For example,when the electrolytic process is the chlor-alkali process, and theelectrolytic cell is a diaphragm cell, such as the one schematicallydepicted in the Figure, a further decrease in the liquid level below thelow-low liquid level can result in exposure of the tops of the cathodesto the gases in the head space of the electrolytic cell, which comprisechlorine gas. When the surface of the cathode is exposed to the headspace atmosphere, the permeability of the diaphragm to hydrogen gasincreases, and the potential exists for leakage of hydrogen gas into thehead space comprising chlorine gas. When the amount of hydrogen gas inthe head space is about 3 weight percent or more, there is a risk ofreaction of hydrogen with chlorine to form hydrogen chloride, i.e. ahydrogen fire fueled by oxidation of hydrogen by chlorine. If thehydrogen concentration is high enough, detonation can also occur. Thus,in some embodiments, a proximity sensor can be positioned on the leveltube at a liquid level at a liquid level of 0 to about 2 inches,specifically 0 to about 1 inch, and more specifically, 0 to about 0.5inch, above the top of the cathode in the electrolytic cell. When asignal is received from a proximity cell so positioned on the level tubeindicating the absence of liquid, immediate steps can be taken to reducethe risk of a hazard such as a hydrogen fire or detonation. These stepsinclude turning off the electric power to the diaphragm cells andpurging the head space of the diaphragm cells with nitrogen, anotherinert gas, or steam. A control system can be designed so that when asignal from a proximity sensor so positioned is received, controls whichcan shut off electrical power to the diaphragm cell, and which can purgethe head space of the diaphragm cell with an inert gas or steam, areautomatically actuated. The design of such control systems are withinthe ability of the skilled person in the art.

A first and second proximity sensor can also be used to provideindications of high and low liquid levels. For example, a first liquidlevel, detected by the first proximity sensor, can be a high liquidlevel, and a second liquid level, detected by the second proximitysensor, can be a low liquid level, each relative to a target liquidlevel determined by the top of the cathode in the electrolytic cell.When the first liquid level represents a high liquid level, the firstliquid level can be about 110 to about 150%, specifically about 110 toabout 140%, and more specifically about 110 to about 130%, of the targetliquid level. The first liquid level can be about 7 to about 12 inches,specifically about 7 to about 11 inches, and more specifically about 7to about 10 inches, above the top of the cathode in the electrolyticcell. When the second liquid level represents a low liquid level, thesecond liquid level can be 0 to about 40%, specifically about 20 toabout 40%, and more specifically about 30 to about 40%, of the targetliquid level. The second liquid level can be 0 to about 4 inches,specifically about 1 to about 3 inches, and more specifically about 2 toabout 3 inches, above the top of the cathode in the electrolytic cell.

The configuration with two or more proximity sensors also allows forredundancy. For example, if the first proximity switch fails, the secondproximity switch can provide a backup signal for low liquid level.Various methods for providing redundancy in liquid level detection, suchas providing a plurality of proximity sensors for each electrolytic celland/or a plurality of communication pathways for each proximity sensorwill be readily apparent to the skilled person in the art.

The proximity sensor is in communication with a control system via acommunication system. The communication system can be a direct electricwire connection or wireless, including infrared. The communicationsystem can be based on an industrial computer network protocol, forexample an industrial Ethernet or a fieldbus. Fieldbus is the name for afamily of industrial computer network protocols standardized underInternational Electrotechnical Commission (IEC) Standard 61158. Fieldbuscomputer network protocols include Foundation Fieldbus H1, ControlNet,Profibus (process fieldbus), P-Net, Foundation Fieldbus HSE (High SpeedEthernet), WorldFIP, and Interbus. Computer network protocols related tofieldbus include AS-Interface (Actuator Sensor Interface, or AS-I), CANbus (controller area network), Interbus, LonWorks, Modbus, Bitbus,CompoNet, SafetyBUS p, Sercos interface, and RAPIEnet. IndustrialEthernet network protocols include EtherCAT, EtherNet/IP, EthernetPowerlink, BACnet, Profinet IO, Profinet IRT, SafetyNET p, Sercos III,TTEthernet, Varan, and RAPIEnet.

In some embodiments, the communication system can be fieldbus, Profibus,industrial Ethernet, or AS-Interface (Actuator Sensor Interface, orAS-i), more specifically an AS-Interface. AS-Interface is designed fornetworking simple field input/output devices, including binary On/Offdevices, in discrete manufacturing and process applications using asingle 2-conductor cable. Binary On/Off devices that can be networkedwith AS-Interface include actuators, sensors, rotary encoders, analoginputs and outputs, push buttons, and valve position sensors. Thecommunication procedure in an AS-Interface is a master-slave method, bywhich the master initiates data exchange with a slave and requires theslave to respond within its defined maximum time. Thus, the AS-Interfacecomprises a network master, a plurality of network slaves, which aresignal input and output modules, a power supply, which powers thenetwork slaves, and which enables communication with the network master,and wiring infrastructure, comprising 2-conductor cables. AS-Interfaceis well-suited as a communication system for proximity sensors.

The control system can be a digital control system (DCS). Examples ofdigital control systems are microcontrollers, application-specificintegrated circuits, programmable logic controllers (PLC, programmablecontroller), microcomputers, and mainframe computers. Microcomputersinclude, for example, workstations, personal computers (PC), portablecomputers, laptop computers, and tablet computers.

The control system is in communication with the proximity sensor, and isresponsive to the proximity sensor. In particular, the proximity sensorgenerates a signal, for example an electric signal, which is indicativeof the presence or absence of liquid at its position on the level tube.This signal is transmitted via the communication system to the controlsystem. The control system can be in communication with a plurality ofproximity sensors positioned to detect the liquid levels in a pluralityof electrolytic cells via the communication system. Based on signalsreceived from the plurality of proximity sensors via the communicationsystem, the control system can indicate the presence or absence ofliquid as well as the identity and location of the electrolytic cell foreach proximity sensor. In this way, the liquid level in a plurality ofelectrolytic cells can be monitored from a remote location. The controlsystem can be programmed to issue an alarm as a function of electrolyticcell location when the liquid level drops out of the range of theproximity sensor. The alarm can be a light alarm, an auditory alarm, ora combination thereof. The alarm can also be in the form of anelectronic signal to a receiver such as a pager.

The control system can be programmed to transform signals form theproximity sensors to a numerical or graphical display of liquid levelfor each electrolytic cell being monitored. The control system can alsobe programmed to store and to display the liquid levels as a function oftime. The programming can be done using commercially available software,for example, LabVIEW, available from National Instruments Corporation ofAustin, Tex. LabVIEW is a graphical programming environment that usesgraphical icons including graphical wires to generate programs in aformat resembling a flowchart.

Methods for detecting liquid level in an electrolytic cell and formaintaining a target liquid level in an electrolytic cell are readilyapparent from the foregoing description. Thus a method for detectingliquid level in an electrolytic cell comprises providing an apparatuscomprising a level tube in fluid contact with an electrolytic cell, aproximity sensor positioned on the level tube below a target liquidlevel to detect the presence or absence of liquid, and a control systemresponsive to the proximity sensor, wherein the control system is incommunication with the proximity sensor via a communication system;wherein the proximity sensor detects the presence or absence of fluid inthe level tube and sends a signal to the control system via thecommunication system; and the control system provides an indication ofliquid level in response to the signal from the proximity sensor.Moreover, a method for maintaining a target liquid level in anelectrolytic cell comprises providing an apparatus comprising a leveltube in fluid contact with an electrolytic cell, a proximity sensorpositioned on the level tube below the target liquid level to detect thepresence or absence of liquid, and a control system responsive to theproximity sensor, wherein the control system is in communication withthe liquid level sensor via a communication system, and provides anindication of liquid level in response to a signal from the proximitysensor; and adjusting the liquid level in the electrolytic cell to thetarget level in response to the signal. Either of the foregoing methodscan further comprise determining a target level for the liquid level,and determining the presence or absence of liquid relative to the targetlevel. Adjusting the liquid level can be performed by the control systemor a human operator.

In another embodiment, a method for maintaining a target liquid level inan chlor-alkali electrolytic diaphragm cell comprises providing anapparatus comprising a level tube in fluid contact with the chlor-alkalielectrolytic cell, a capacitive proximity sensor adjustably mounted onthe level tube to detect the presence or absence of a brine at apredetermined position on the level tube, a support for positioning andmounting the capacitive proximity sensor on the level tube, and adigital control system responsive to the capacitive proximity sensor,wherein the digital control system is in communication with thecapacitive proximity sensor and indicates the presence or absence of thebrine at the predetermined position on the level tube in response to asignal from the proximity sensor; and adjusting the brine level in thechlor-alkali electrolytic cell to the target level in response to thesignal. For example, a method for maintaining a target liquid level in achlor-alkali electrolytic diaphragm cell comprises detecting with acapacitive proximity sensor the presence or absence of a brine at apredetermined position on a level tube in fluid contact with thechlor-alkali electrolytic cell, sending a signal from the capacitiveproximity sensor to a digital control system in communication with thecapacitive proximity sensor indicating the presence or absence of thebrine at the predetermined position on the level tube; generating asignal from the control system indicating the presence or absence of theliquid at the predetermined position on the level tube; and adjustingthe brine level in the chlor-alkali electrolytic cell to the targetlevel in response to the signal from the control system.

The apparatus for detecting a liquid level in an electrolytic cell canalso be used to monitor inspection rounds made by an operator of theelectrolytic cell. Operators can make inspection rounds at fixedintervals of time, for example at 30-minute intervals, to determineliquid levels by visual inspection of level tubes. When proximitysensors are positioned on the level tubes, the operator can inspect theproximity sensor at the same time. The operator can temporarily adjustthe position of the proximity sensor above the actual liquid level togenerate a signal for the absence of liquid, and then replace it to itsoriginal position. In this way, an inspection log based on briefindications of the absence of liquid at fixed time intervals can begenerated by the digital control system.

The control system can also be programmed to automatically adjust thefluid level in the electrolysis cell in response to indications of lowor high fluid levels detected by the proximity sensors. In this method,the valves and/or pumps that control the flow of liquid in and out ofthe electrolytic cell are actuated valves and pumps, capable of beingturned on and off, and adjusted, based on signals received from thecontrol system via a control bus. Apparatus and methods for setting upautomatic control of liquid level in an electrolytic cell will beapparent to the skilled person in the art based on the description ofthe apparatus and methods herein.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions, or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

What is claimed is:
 1. An apparatus for detecting a liquid level in anelectrolytic cell, the apparatus comprising a level tube in fluidcontact with the electrolytic cell; a proximity sensor positioned todetect the presence or absence of liquid at a predetermined level in thelevel tube; and a control system responsive to the proximity sensor,wherein the control system is in communication with the proximity sensorvia a communication system.
 2. The apparatus of claim 1, wherein theposition of the proximity sensor is adjustable along the vertical axisof the level tube.
 3. The apparatus of claim 1, wherein the proximitysensor is a capacitive proximity sensor.
 4. The apparatus of claim 5,wherein the capacitive proximity sensor has a fixed capacitance fordetecting the presence or absence of liquid in the level tube.
 5. Theapparatus of claim 5, wherein the capacitive proximity sensor has anadjustable capacitance tunable to a value effective to detect thepresence or absence of liquid in the level tube.
 6. The apparatus ofclaim 5, wherein the capacitive proximity sensor has a flatconfiguration and an adjustable capacitance.
 7. The apparatus of claim1, wherein the communication system is a digital communication bus. 8.The apparatus of claim 1, wherein the control system issues a signal toan operator in response to a signal from the proximity sensor.
 9. Theapparatus of claim 1, wherein the liquid is an electrolyte.
 10. Theapparatus of claim 1, wherein the electrolytic cell is a chlor-alkalielectrolysis diaphragm cell.
 11. The apparatus of claim 10, wherein theliquid is brine.
 12. The apparatus of claim 1, wherein the proximitysensor detects a low liquid level, and wherein the low liquid level is 0to about 70 percent of a target liquid level.
 13. The apparatus of claim12, wherein a target liquid level is about 6 to about 8 inches above thetop of a cathode in the electrolytic cell; and wherein the low liquidlevel is 0 to about 4 inches above the top of the cathode in theelectrolytic cell.
 14. The apparatus of claim 1, comprising a firstproximity sensor and a second proximity sensor, wherein the firstproximity sensor detects a first liquid level, and the second proximitysensor detects a second liquid level below the first liquid level. 15.The apparatus of claim 14, wherein the first liquid level represents alow liquid level and the second liquid level represents a low-low liquidlevel, and wherein the first liquid level is about 40 to about 80percent of a target liquid level, and the second liquid level is 0 toabout 40 percent of the target liquid level.
 16. The apparatus of claim14, wherein a target liquid level is about 6 to about 8 inches above thetop of a cathode in the electrolytic cell; wherein the first liquidlevel represents a low liquid level and the second liquid levelrepresents a low-low liquid level; and wherein the first liquid level isabout 3 to about 5 inches above the top of the cathode in theelectrolytic cell, and the second liquid level is about 2 to about 3inches above the top of the cathode in the electrolytic cell.
 17. Theapparatus of claim 14, wherein the first liquid level represents a highliquid level and the second liquid level represents a low liquid level,and wherein the first liquid level is about 110 to about 130 percent ofa target liquid level, and the second liquid level is 0 to about 40percent of the target liquid level.
 18. The apparatus of claim 14,wherein a target liquid level is about 6 to about 8 inches above the topof a cathode in the electrolytic cell; wherein the first liquid levelrepresents a high liquid level and the second liquid level represents alow liquid level; and wherein the first liquid level is about 7 to about10 inches above the top of the cathode in the electrolytic cell, and thesecond liquid level is about 2 to about 3 inches above the top of thecathode in the electrolytic cell.
 19. An apparatus for detecting a brinelevel in a chlor-alkali electrolytic cell, the apparatus comprising alevel tube in fluid contact with the chlor-alkali electrolytic cell; acapacitive proximity sensor adjustably mounted on the level tube todetect the presence or absence of brine at a predetermined position onthe level tube; a support for positioning and mounting the capacitiveproximity sensor on the level tube; and a digital control systemresponsive to the capacitive proximity sensor, wherein the digitalcontrol system is in communication with the capacitive proximity sensor.20. A method for maintaining a target liquid level in an electrolyticcell, the method comprising providing an apparatus comprising a leveltube in fluid contact with an electrolytic cell, a proximity sensorpositioned on the level tube at a target liquid level to detect thepresence or absence of a liquid, and a control system responsive to theproximity sensor, wherein the control system is in communication withthe proximity sensor via a communication system, and indicates thepresence or absence of the liquid at the target liquid level on thelevel tube in response to a signal from the proximity sensor; andadjusting the liquid level in the electrolytic cell to the target liquidlevel in response to the signal.
 21. The method of claim 20, the methodfurther comprising detecting with the proximity sensor the presence orabsence of the liquid at the target liquid level on a level tube;sending a signal from the proximity sensor to the control system incommunication with the proximity sensor indicating the presence orabsence of the liquid at the target liquid level on the level tube;generating a signal from the control system indicating the presence orabsence of the liquid at the target liquid level on the level tube; andadjusting the liquid level in the electrolytic cell to the target liquidlevel in response to the signal from the control system.
 22. The methodof claim 20, wherein the proximity sensor comprises a display.
 23. Themethod of claim 20, wherein the proximity sensor is a capacitiveproximity sensor.
 24. The method of claim 23, wherein the capacitiveproximity sensor has a fixed capacitance for detecting the presence orabsence of liquid in the level tube.
 25. The method of claim 23, whereinthe capacitive proximity sensor has an adjustable capacitance tunable toa value effective to detect the presence or absence of liquid in thelevel tube.
 26. The method of claim 23, wherein the capacitive proximitysensor has a flat configuration and an adjustable capacitance.
 27. Themethod of claim 20, wherein the communication system is a digitalcommunication bus.
 28. The method of claim 20, wherein the controlsystem issues a signal to an operator when the liquid is present orabsent at the target liquid level on the level tube.
 29. The method ofclaim 20, wherein the liquid is an electrolyte.
 30. The method of claim20, wherein the proximity sensor detects a low liquid level, and whereinthe low liquid level is 0 to about 70% of the target liquid level. 31.The method of claim 30, wherein the target liquid level is about 6 toabout 8 inches above the top of a cathode in the electrolytic cell; andwherein the low liquid level is 0 to about 4 inches above the top of thecathode in the electrolytic cell.
 32. The method of claim 20, whereinthe apparatus comprises a first proximity sensor and a second proximitysensor, wherein the first proximity sensor detects a first liquid level,and the second proximity sensor detects a second liquid level below thefirst liquid level.
 33. The method of claim 32, wherein the first liquidlevel represents a low liquid level and the second liquid levelrepresents a low-low liquid level, and wherein the first liquid level isabout 40 to about 80 percent of the target liquid level, and the secondliquid level is 0 to about 40 percent of the target liquid level. 34.The method of claim 32, wherein the target liquid level is about 6 toabout 8 inches above the top of a cathode in the electrolytic cell;wherein the first liquid level represents a low liquid level and thesecond liquid level represents a low-low liquid level; and wherein thefirst liquid level is about 3 to about 5 inches above the top of thecathode in the electrolytic cell, and the second liquid level is about 2to about 3 inches above the top of the cathode in the electrolytic cell.35. The method of claim 32, wherein the first liquid level represents ahigh liquid level and the second liquid level represents a low liquidlevel, and wherein the first liquid level is about 110 to about 130percent of the target liquid level, and the second liquid level is 0 toabout 40 percent of the target liquid level.
 36. The method of claim 32,wherein the target liquid level is about 6 to about 8 inches above thetop of a cathode in the electrolytic cell; wherein the first liquidlevel represents a high liquid level and the second liquid levelrepresents a low liquid level; and wherein the first liquid level isabout 7 to about 10 inches above the top of the cathode in theelectrolytic cell, and the second liquid level is about 2 to about 3inches above the top of the cathode in the electrolytic cell.
 37. Amethod for maintaining a target brine level in a chlor-alkalielectrolytic cell, the method comprising providing an apparatuscomprising a level tube in fluid contact with the chlor-alkalielectrolytic cell, a capacitive proximity sensor adjustably mounted onthe level tube to detect the presence or absence of brine at the targetbrine level on the level tube, a support for positioning and mountingthe capacitive proximity sensor on the level tube, and a digital controlsystem responsive to the capacitive proximity sensor, wherein thedigital control system is in communication with the capacitive proximitysensor and indicates the presence or absence of the brine at the targetbrine level on the level tube in response to a signal from thecapacitive proximity sensor; and adjusting the brine level in thechlor-alkali electrolytic cell to the target brine level in response tothe signal.
 38. The method of claim 37, further comprising detectingwith a capacitive proximity sensor the presence or absence of brine atthe target brine level; sending a signal from the capacitive proximitysensor to the digital control system indicating the presence or absenceof the brine at the target brine level on the level tube; generating asignal from the digital control system indicating the presence orabsence of the brine at the target brine level on the level tube; andadjusting the brine level in the chlor-alkali electrolytic cell to thetarget brine level in response to the signal from the digital controlsystem.